Method for the production of platinum coated titanium anodes



United States Patent 3,177,131 METHOD FOR THE PRODUCTIGN 0F l LA'liNUl /i QOATED TITANIUM ANODES Clifford Hyde Angeli, Sutton Coldfield, and Marcel George Deriaz, Widues, England, assignors to Imperial Chemical Industries Limited, London, England, a corporation of Great Britain No Drawing. Filed Aug. 12, 1963, Ser. No. 391,659 Claims priority, application Great Britain, Apr. 27, 1959, 14,254/59; Aug. 18, 1962, MAW/62 3 Claims. (Cl. 204-98) This application is a continuation-in-part of our U.S. application Serial No. 20,811, filed April 8, 1960, now abandoned.

This invention relates to anode assemblies comprising a surface of platinum borne on a supporting structure of titanium and a particularly advantageous method of producing such assemblies; it relates further to the use of assemblies prepared in this special manner as anodes in the electrolysis of brine.

One object of this present invention is the provision of improved anode assemblies comprising a surface of platinum borne on a supporting structure of titanium which have especially useful properties when used as anodes in the electrolysis of aqueous solutions of alkali metal chlorides.

A second object is the provision of a special method of producing such anode assemblies the advantageous properties whereof arise from and are peculiar to the method of production.

A further object is the provision of an improved procedure of electrolysing aqueous solutions of alkali metal chlorides characterised in that there is used as an anode in such procedure an assembly comprising a surface of platinum borne on a supporting structure of titanium, the said assembly having been made by the aforesaid special method of production whereby distinct advantages are secured in the electrolysis procedure, particularly in respect of the voltage needed to operate the electrolytic cell, especially at relatively high current density.

These and other objects are achieved by the invention which will now be described.

It is already known to make assemblies useful as electrodes in electrolytic processes by coating various base metals with noble metals such as platinum, rhodium and iridium and some such assemblies have been proposed for use as anodes in the electrolytic production of per compounds. Furthermore it has been suggested that the so-Called valve-acting metals, namely, beryllium, aluminiurn, magnesium, niobium, tantalum, zirconium and titanium may thus be used as a supporting structure for a noble metal anode in a corrosive electrolyte because the interaction with the electrolyte of any such base metal as is exposed (in those places where the coating of noble metal is lacking or is defective, for example, porous) forms an adherent oxide film which then protects the underlying base metal from further attack.

Thus, for example, in U.S. specification No. 2,719,797 of Rosenblatt there is described a method of providing an adherent coating of a platinum metal on a tantalum support which comprises first providing a thin coating of the platinum metal on the tantalum base and thereafter heating the composite structure to a temperature of 800 to 1400 C. to bring aboutbonding of the two metals by inter-diffusion or alloying in the interfacial layers. The specification states that the initial coating with platinum may be brought about by treating the tantalum surface with a solution containing a reducible platinum compound such as chloroplatinic acid together with other conventional adjuvants and subsequently heating at a temperature (approximately 250 C.) at which the chloroplatinic acid is decomposed to form metallic platinum and it is explained that a platinum coating formed simply in this Way is inadequately adherent and if the so coated tantalum is used as an anode in a corrosive electrolyte, for instance in electrolysing sodium chloride solutions to make chlorine or chlorates, the coating is very rapidly stripped off so that the anode becomes useless. It is stated that this defect is only cured by applying the controlled diffusion or alloying step which is the essential object of the invention; this involves a subsequent heating of the coated tantalum at high temperatures in the range 800- 1400" C.

In co-pending U.S. application Serial No. 780,882, filed December 16, 1958, it is taught that an anode which is a surface of platinum borne on a supporting structure of titanium or an alloy consisting essentially of titanium is of outstanding value in that it overcomes most of the difiiculties which are met with in designing and constructing an anode for use in cells for the electrolysis of aqueous solutions of alkali metal chlorides to make chlorine and caustic alkali. It is stated that the platinum coating on the titanium support may be provided in a number of Ways, one of them being painting the titanium support with a conventional platinising solution and subsequently heating in the manner practised in the ceramics industry.

Likewise in co-pending U.S. application Serial No. 796,- 856, filed March 3, 1959, there is described a particular form of multi-cell adaptable for the electrolytic production of chlorine and caustic alkali, hypochlorite or chlorate from aqueous solutions of alkali metal chlorides comprising a plurality of unit cells separated one from another by means of partitions composed of titanium or an alloy consisting essentially of titanium which partitions may each have one face (or a portion thereof) coated with a thin layer of platinum which can then function as the anode of that unit cell. It is also taught in that application that the coating of titanium can be suitably applied to the titanium partition electrolytically or chemically, for instance by applying a conventional platinumbearing preparation and subsequently heating to deposit a thin film of the metal.

Arising from the teachings of the various documents cited above it was considered, until the date of our copending parent application Serial No. 20,811 of which the present application is a continuation-in-part, that the best Way to coat with platinum a titanium support intended for use as an anode in an electrolytic cell was to deposit the platinum electrolytically on the titanium. High temperature (800-1400 C.)' firing to produce diffusion or alloying of a paint-platinised tantalum support had indeed been advocated by Rosenblatt as a means of making an anode useful in making electrolytic chlorine or chlorates but he had also categorically stated that without the final high temperature firing the coating readily stripped from the base and the anode was useless. Furthermore, his teaching was specific to tantalum. Thermal reduction of a coating deposited on tantalum from a solution of a reducible platinum compound and subsequent firing of the platinum deposit in a reducing atmosphere had also been taught by Tirrell to give an anode useful in an electrodialysis unit for demineralising brackish water, but Tirrell had also subsequently stated that a similar procedure applied to a titanium support gave an anode which was inoperative for that purpose.

In contrast to these teachings we have now found that for assemblies comprising a surface of platinum borne on a titanium support and intended for use as anodes in the electrolysis of aqueous solutions of alkali metal chlorides, better results are obtained, particularly in respect of the voltage needed to operate the electrolytic cell at relatively high current density, if the coating of platinum metal (or Patented Apr. 6, 1965 thelast such coating if a multicoat system is adopted) is derived by heating in an oxiding atmosphere to a temperature in the range 350550 C. a film or coatingapplied tothe titanium support of a platinum-bearing preparation as hereinafter more particularly defined. We prefer, in fact, to'he'at the coated titanium in air at a temperature falling in the range 350,500 C.

We have found that if the coating. is heated at temperatures above 550C, e.g., at 600 C. in an oxidizing atmosphere' for 5 minutes, the assembly, ifused as ananode in a mercury cell for the electrolysis of brine, requires voltages which for a given current density slowly increase with time over a period of five months, owing to the slow development of overvoltage. It is believed that this. is because during the heating the platinum particles sinter together too much and also diffuse into the titanium. If the coating is heated attemperatures still higher, e.g., 800 C., the electrical contact between the platinum layer, and the titanium is poor and the assembly passes little current if used as an anode in mercury cells.

p If the coating of platinum-bearing preparation is heated in an oxidising atmosphere to a temperature below 350 C.,,for.instance,for 15 minutes, the organic matter of the said preparation .is carbonised to some extent and not completely volatilised and burnt. The resulting deposit of platinum and carbon has poor adhesion to the underlying titanium, possibly on account of the presence of carbon,.but possibly also because the temperature of heating is too low for the required degree of sintering of the platinum metal particles to one another and to the underlying. titanium. The resulting assemblies, if used as anodes in brine electrolysis have a shorter life than those heated at 350 550 C., e.'g., only half the life of those heated at 350 to 500 C. in an oxidising atmosphere.

According to the present invention therefore. a method for the production of an anode assembly comprising a surface of platinum on a titanium support comprises applying to the titanium support at least one coating of a platinum-bearing preparation comprising a thermally reducible platinum compound in an organic vehicle and which on heating to a temperature between 350 and,

temperature range 500-550 C. is within the scope of the,

invention and also produces useful assemblies.

A further feature of the invention is a process for making chlorine and caustic alkali by electrolysingaqueous solutions of alkali metal chlorides wherein there is used as the anode of thecell an. assembly comprising a surface coating of platinum on a titanium support which coating has been made by applying to the support a primary coating of a platinum-bearing preparation comprising a thermally reducible platinum compound in an organic vehicle, drying said primary coating and firing the same by heating in anoxidising atmosphere to a temperature in the range 350'550 C.

In this specification and claims the term titanium includes not only titanium itself but also alloys based 'on titanium and having .anodic polarisation properties comparable with those of titanium. Examples of the latter are titanium-zirconium alloys containing up to 14% of zirconium, alloys. of titanium with up to of a platinum metalsuch as platinum, rhodium or iridium and alloys of titanium with niobium or tantalum containing up to of the alloying constituent.

Platinum-bearing preparations comprising solutions of a thermally-reducible platinum compound in an organic vehicle are themselves Well known, being conventionally used for purposes, of metallic decoration in the ceramic arts. They are usually made by dissolving a thermally decomposable platinum compound such as chloroplatinic acid or bromoplatinic acid in a volatile solvent and adding thereto a mixture of a resin with terpineol, acetoacetic ester or an essential oil. It is usually heldthatin the solution or during the initial drying of the primary coating resinates or sulpho-resinates of platinum are. formed which are burned off in the subsequent firing step to form the ultimate deposit of platinum metal. Such. platinumbearing preparations and their utilisation in the ceramic art are describecL'for instance, in Platinum Metals Review for October 1958 where it is indicated that the customary firing temperatures are in the range 5 50 -800 C. We have found, howeven that for the purposes of the present invention lower firing temperatures are needed, namely in the range 350-550 C. and that the firing should be carried out in an oxidising atmosphere. One

such platinum-bearing preparation suitable for use in the method of the. present invention is that sold under the trade. name Hanovia 'Liquid Bright Platinum Grade 05-X. This product comprises a heat decomposable platinum compound which decomposes in the range of 350 and 550 C., dissolved in an appropriate volatile organic solvent. Other suitable preparations may be made by dissolving in a volatile organic solvent a platinum compound which. decomposes on heating to yield platinum metal and volatile b y-p roducts, for instance, chloroplatinic or bromoplatinic acid, and adding to the solution a mixture of a resinand terpineol, ethyl .acetoacetate or an essential oil, for instance oil ofv cloves, oil of turpentine, oil of lavender. For example, a solution of-chloroplatinic acid in ethyl alcohol or in isopropyl. alcohol may be added to an equal volumeof a mixture in equal parts of abietic, acid and oil of clovesr We have spoken above simply of platinum-bearing preparations but it is to be understood that we do not intend the term to be limited strictly to preparations containing platinum as the sole noble; metal. Certain of the commercial preparations contain also minor amounts of other metals of the platinum group, particularly rhodium and/ or iridium. Furthermore. such. preparations can readily be made by the methods described above, adding appropriate amounts of readily decomposablerhodium or iridium salts to'the platinum salt. therefore that the term .pla'tinum in this specification and claims includes, where the context permits, also platinum containing minor proportions of rhodium and/ or iridium-one such example is an alloy consisting of 70% latinum, 30% iridium.

We have found that the optimum temperature for firing the coating of platinum-bearing preparation in an oxidising atmosphere, to obtain anode coatings which in chlorine cells operate at thelowestcell voltage. for long periods depends to some extent on the type of platinum-bearing preparation 7 used. For example, coatings of Hanovia Liquid. Bright Platinum Grade 05-X provide the most suitable assemblies for use .asanodes'in the electrolysis of brine Whenfi're'd at 40055'05C., and a temperature of 4-76 C. is preferred, whereas a preparation consisting of chloroplatini'c acid 1 gram, ethyl alcohol 10 mL, and an equal volume of a 1:1 mixture of oil-of'cloves and abietic acid, provides the most suitablea-ssemblies when fired at 350 500 C., and a temperature of- 400 C. is preferred. The optimum firingtemperature for any particular platinum-bearing preparation can easily be determined by simple trial as shown hereinafter. 1

Preferably the platinised surface is formedby superimposing a number of said depositsby repeating the operation of coatingv with a platinum-bearing preparation ofthe aforesaid kind, drying each. coating. and subsequen'tl'y firing by heating in an oxidising atmosphere, for

example, by heating in air for 10 minutes to a temperature lyingbetween 350 and 550 C., particularly between- 350 and 500 C. Furthermore, preferably the first coat- It is to be understood.

greater ing is applied to a surface of a titanium support which has first beentreated to remove therefrom the surface skin consisting chiefly of oxide, as for example according to the method described in British patent specification No. 758,013.

Also, if desired, the titanium support, preferably first cleaned by removal of the oxide skin as just indicated, can first be given a surface coating of an electrolytic deposit of platinum before the first coating of a platinumbearing operation is applied. Such an electro-deposit can conveniently be made by conventional methods using the known alkaline hexahydroxyplatinate electroplating bath.

For instance, in accordance with one embodiment of the invention the parts of the surface of the titanium support to be provided with a platinum coating are degreased, then etched for 4 days in analytical reagent quality concentrated hydrochloric acid, allowed to dry, then painted with a platinum-bearing preparation comprising chloroplatinic acid, ethyl alcohol, oil of cloves and abietic acid in the proportions described above, the resulting coating dried for example by infra-red radiation, and the dry coating then fired, in air, in an oven at a temperature in the range 375 -475 C. for minutes. The painting,

, drying and firing operations are then repeated as often as required to give a platinum coating of desired thickness. For instance 5-15 coatings of the platinum-bearing preparation may suitably be appled,-- with the necessary intervening drying and firing of each coating.

The number of coatings required to build up a desired thickness of platinum depends on the platinum content of the coating preparation and on the thickness of each applied coating. For a given type of coating preparation the maximum usable platinum content is limited in the main by the viscosity of the mixture. We prefer to employ platinum-bearing preparations containing the equivalentof 2-9% by weight of platinum. Even smaller platinum contents are permissible, but they necessitate the application of an excessive number of coatings.

The coating of platinum left after firing is finely divided and has a light grey mat fininsh. When operating as an anode it is very dark grey or black. It is presumably this state of subdivision of the platinum coating which permits the assemblies of the present invention to behave in so satisfactory a manner for instance as anodes in a mercury cell for the electrolysis of brine.

As already indicated the platinum-bearing preparation is preferably applied to etched titanium since this gives a still more adherent coating of platinum. This is particularly useful if each anode in each cell of a series of mercury cells is an assembly produced according to the invention, because in such a series it is desirable to be able to short out any one cell temporarily by connecting its anode to its cathode While leaving the rest of the series operating. This method of shorting out a cell causes hydrogen to be momentarily discharged from the anode. It is found that these assemblies are particularly well able to withstand the momentary cathodic polarisation thus generated.

Assemblies produced according to the method of the invention are particularly well adapted to serve as improved anodes for use in electrolytic cells for the manufacture of chlorine and caustic alka l-i by the electrolysis of aqueous solutions of alkali metal chloride, for instance as an anode of the kind which is broadly described in 00- pending application Serial No. 780,882 (filed December 16, 1958) or as an impermeable barrier provided with an anodic surface coating of platinum on one side thereof in a multi-electrolytic cell as is broadly described in copending application Serial No. 796,856 (filed March 3, 1959). Assemblies produced according to the method of the invention may also be fashioned so as to permit them to serve as improved electrodes in electrolytic cells for the manufacture of alkali metal chlorates, for instance as bipolar electrodes in a multi-electrolytic cell 6 as claimed in co-pending application Serial No. 114,354 (filed June 2, 1961).

The following examples illustrate the production of assemblies comprising a surface of platinum on a titanium support within the scope of the invention and the operation of such assemblies as anodes in mercury cells and diaphragm cells for the production of chlorine and caustic soda. Since the titanium supports were in the form of expanded titanium sheet, of which the true surface area was not accurately known, the platinum loading of the titanium surface and the cell current density were calculated in terms of the overall geometrical area of the anode, and for ease of measurement this was taken as the area of the associated cell cathode, which was essentially planar.

Example 1 A comparison at given current densities of the cell voltages of a mercury cell having as anode an electrodeposited platinum coating on an expanded sheet of titanium metal with a mercury cell of the same dimensions having as anode a structure produced according to the method of the present invention consisting of fired coatings of a platinum-bearing preparation on an expanded sheet of titanium was carried out as follows.

An anode was prepared by electrodepositing 100 grams of platinum per square metre, calculated on the cell cathode area, on an expanded sheet of titanium which had been degreased, etched for 4 days in analytical reagent quality concentrated hydrochloric acid and subsequently dried. An anode was also prepared by applying to a similarly degreased, etched and dried expanded sheet of'titanium two coatings of a platinum-bearing preparation of platinum resinate in an essential oil (oil of cloves) containing the equivalent of 8% of platinum and firing each coating in air at 550 C. to give a deposit of platinum of 25 grams er square metre of cathode area.

Brine of 23% (w./w.) sodium chloride at C. was electrolysed in each of the two mercury cells. The relationship between current density and voltage of the mercury cell for current densities exceeding 2 ka./m. is expressed by the straight line equation V=A bC where V is voltage of the cell.

A is the intercept on the voltage axis obtained by extrapolating the straight line backwards.

C is current density (lea/m b is the slope of the curve.

For the mercury cell having the electrodeposited platinum surface as anode, by plotting voltage against current density it was found that For the mercury cell having the platinum coating derived from the heated platinum-bearing preparation as anode, by plotting voltages against current density it was found that Hence there was an advantage of 0.3 v. at all current densities exceeding 2 ka/m. for a structure according to the invention as anode as against an anode with an electrodepositied platinum surface.

The rate of loss of platinum from the two types of anode when used in a mercury cell for the electrolysis of brine was of the same order, namely, about 1 gram of platinum per ton of chlorine produced when the mercury cell operated at 4 ka./m.

Example 2 A comparison at given current densities of the cell voltage of a mercury cell having as anode an electrodeposited platinum coating on an expanded sheet of titanium with voltages of mercury cells of the same dimen- An anode was again prepared by electrodepo'siting 100 grams of platinum per square metre, calculated on the cell cathode area, on an expanded sheet of titanium which had been degreased, etched for 4 days in analytical reagent quality concentrated, hydrochloric acid and subsequently dried.-

A series of anodes was also prepared by superimposing on each of a number of similar and similarly degreased,

etched and dried expanded sheets of titanium ten coatings of a commercial platinum-bearing preparation of platinum resinate in an essential oil (oil of cloves) containing the equivalent of about 8% by weight of platinum and firing each'coating in air to give on each expanded titanium sheet adeposit of platinum of 60 grams per square metre of cathode area. Each of the superimposed coatings was fired at the same temperature, a diiferent predetermined firing temperature lying between 475 and 550 C. being employed for each coated titanium specimen.

Brine of 23% (w./w.) sodium chloride at 60 C. was ele'ctrolysed using one of the prepared anodes in each mercury cell. The relationship between current density p For the mercury cell having electrodeposited platinum on the expanded sheet titanium support as anode, by plotting voltage against current density it was again found that For the mercury cells each having the platinum coating derived from the fired platinum-bearingpreparation on the expanded sheet titanium support as anode, by plotting voltages against current density it was found after continuous operation for 23 weeks at a current density of 4' Hence, for example, for a firing temperature for the platinum-bearing preparation of 475, 500, 525 and 550 C. there was an advantage of 0.28 v., 0.23 v., 0.18 v. and 0.08 v. respectively, at all current densities exceeding 2 lea/m for a structure according to the invention as against an anode of the electrodeposited platinum on an expanded sheet titanium support.

It is also seen that anodes prepared by firing the applied platinum bearing preparation at the lower temperatures, e.g., 475 and 500 C. have an advantage in lower operating voltages for long periods of time over those prepared by firing the applied platinum-bearing preparation at the higher temperatures, e.g., 525 and 550 C.

The rate of loss of platinum from the various typesof anodes used in this test was approximately the same, namely about 1 gram of platinum per ton of chlorine pro duced when the mercury cells operated at 4 lea/m Example 3 A platinum-bearing preparation containing about 3% by weight of platinum was prepared as follows. 5 g. abietic acid were mixed with 5 g. eugenol, heated to 150 C. and cooled to give a light'brown syrup. 1 g. chloroplatinic acid was dissolved in 2 ml. absolute ethyl alcohol and mixed with the syrup.

Two expanded sheets of titanium prepared asin Example 1 by degreasing, etching and drying were each given fourteen coatings of the above platinum-bearing preparation, each coating being fired in air, .at 350 C. on

one. titanium specimen and at 500 C..on the other.

Each of the coated sheets of titanium was employed as the anode in an experimental mercury cell immediately after it had been prepared, and the measurement of cell voltage for a range of current densities. exceeding 2 ka./m. showed the following characteristics. The deposit of platinum was about 30 g. per square metre, calculated on the cell cathode area.

3. l0-I-0. 20 C 3. 15+0. 20 G Each of these anodes was then transferred to an experimental diaphragm cell producing chlorine and caustic soda operating at a current density of 6' kit/n1 with brine containing 18% NaCl w./w. at 70 C. The rate of platinum loss from anode W over thefirst 26 weeks of its life was 0.16 g./ton of chlorine produced and the rate of loss-from anode X over 26 Weeks was 0.22 g./ton

A platinum-bearing preparation was made up as follows. 2 g. abietic acid were mixed with 2 g. eugenol. To this was added a solution of 1.216 g. chloroplatinic acid in 1.5 ml. ethyl alcohol. The resulting mixture was too viscous and was therefore diluted with half its volume of cugeuo'l before use, the platinum content then being about 5% by weight.

Two expanded sheets of titanium prepared as in Example 1 by degreasing, etching anddrying were each given thirteen coatings of the above platinum-bearing preparation, each coating being fired in air at 350 C. on one titanium specimen and at 475 C. on the other.- When tested as anodes in experimentalmercury cells as inExarnple 2, the specimens showed the following characteristics at current densities exceeding 2 kaJ/mF. The deposit of platinum was about 30 g. per square metre, calculated on the cell cathode area.

Anode I Firing gemp Cell Voltage Each of these anodes wasthen operated in experimental diaphragm cells as in Example 2. The rate of platinum loss from anode Y over the first 26 weeks of its life was 0.1 g./ton of chlorine produced and the rate of loss of platinum from anode Z over 26 weeks was approximately 0.2 g./ ton of chlorine produced.

Example 5 Nine platinised'titaniu'm'anodes were fitted in a commercial mercury cell making chlorine and caustic soda by the electrolysis of brine. Each anode was made by degrea'sing, etching and-drying an expanded titanium sheet as described in Example 1 and subsequently applying 20 coatings of a commercial platinum-bearing preparation consisting of a. platinum resinate in an essential oil (oil of cloves) containing the equivalent of 8% of platinum, each coating in turn being dried and fired in air at 475 C. for 10 minutes. The total platinum deposit was 40 g./m. on each faceof the titanium sheet.

Brine containing 23% w./w'. of sodium chloride was electrolyzed continuously in thecell for nine months at a temperature of approximately 60? C. and at current densities varying from 2.9 to [5.4 ka./m. calculated on the cathode area of the cell. The average overvoltage for the nine anodes was initially 0.060'volt and this had.

risen only to 0.067 volt at the end of the nine months ClO 9 running. The rate of platinum loss, measured by a radioactive thickness gauge, was 0.36 g./ton of chlorine produced.

Anodes made according to the teachings of this present invention have been directly compared with and shown to be superior to anodes made according to the teachings of Rosenblatt in US. specification No. 2,719,797, referred to above. In a direct comparison of the anodes when used in a cell for making chlorine and caustic alkali by electrolysis of an aqueous solution of sodium chloride, an anode made by applying a platinum coating of 10 grams per square metre to tantalum by painting and firing in the manner acknowledged by Rosenblatt as the prior art practice failed under the conditions of test (high current density) after only 20 minutes; the platinum coating stripped off the tantalum support. An anode made by Rosenblatt himself, following the teachings of his own specification (high temperature after-treatment) failed under the same test conditions after 23 hours. An anode made according to the present invention (10 grams per square metre of platinum applied by painting and firing in an oxidizing atmosphere at a temperature of 475 C.) was entirely unaffected after 112 hours. Moveover anodes made by this procedure have been in service in commerical brine cell installations (both mercury cells and diaphragm cells) for periods of more than 2 years without failure of either the coating of platinum or the titanium support.

What we claim is:

1. A method for the production of an anode assembly comprising a surface of platinum on a'titanium support which comprises applying to the titanium support at least one coating of a platinum-bearing preparation comprising a thermally reducible platinum compound in an organic vehicle and which on heating to a temperature between 350 and 550 C. produces a deposit consisting essentially of platinum, said platinumcompound being selected from the group consisting of chloroplatinic acid, bromoplatinic acid and platinum resinates, drying each coating and then firing the same by heating in an oxidising atmosphere to a temperature between 350 and 550 C. to form said deposit consisting essentially of platinum.

2. A method according to claim 1, wherein the heating to form said deposit consisting essentially of platinum is carried out in air.

3. A method according to claim 1 wherein the heating to form said deposit is carried out in air and at a temperature between 350 and 500 C.

4. A method for the production of an anode assembly comprising a surface of platinum on a titanium support which comprises applying to the titanium support a plurality of coatings of a platinum-bearing preparation containing 2% to 9% by weight of platinum in the form of i 5. A method according to claim 4 wherein before coating with the platinum-bearing preparation, the surface of the titanium support has been treated to remove therefrom the surface skin consisting chiefly of oxide.

6. A method according to claim 4 wherein before coating with the platinum-bearing preparation the surface of the titanium support has been first etched and subsequently electrolytically provided with a surface coating of platinum.

7. A process for the manufacture of chlorine and caustic alkali, which comprises applying to a titanium support at least one coating of a platinum-bearing preparation comprising a thermally reducible platinum compound in an organic vehicle and which on heating to a temperature between 350 and 550 C. produces a deposit consisting essentially of platinum, said platinum compound being selected from the group consisting of chloroplatinie acid, brornoplatinic acid and platinum resinates, drying each coating and then firing the same by heating in an oxidising atmosphere to a temperature between 350 and 550 C. toform said deposit consisting essentially of platinum and electrolysing an aqueous solution of an alkali metal chloride in an electrolytic cell using said platinum coated titanium support as the anode.

8. A method for the production of an anode assembly comprising a surface of platinum on a titanium support which comprises applying to the titanium support at least one coating of a platinum-bearing preparation comprising a thermally reducible platinum sulphoresinate in an organic vehicle and which on heating to a temperature between 350 and 550 C. produces a deposit consisting essentially of platinum, drying each coating and then fir-- ing the same by heating in an oxidising atmosphere to a temperature between 350 and 550 C. to form said deposit consisting essentially of platinum.

References Cited by the Examiner UNITED STATES PATENTS 2,719,797 10/55 Rosenblatt 204-290 2,825,682 3/58 Missel 204-382 2,865,785 12/58 Coles 117-46 2,955,999 10/60 Tirrell 204-47 2,987,453 6/61, Du Rose 204-290 X 2,998,359 8/61 Anderson 204-383 3,022,177 2/62 Fitch 117-130 3,092,504 6/63 Langley 117-130 3,096,272 7/63 Beer 204-290 3,117,023 1/64 Tirrell 117-130 FOREIGN PATENTS 877,901 9/61 Great Britain.

OTHER REFERENCES Chemical Age, January 1959, p. 9.

WINSTON A. DOUGLAS, Primary Examiner.

MURRAY TILLMAN, Examiner. 

7. A PROCESS FOR THE MANUFACTURE OF CHLORINE AND CAUSTIC ALKALI, WHICH COMPRISES APPLYING TO A TITANIUM SUPPORT AT LEAST ONE COATING OF A PLATINUM-BEARING PREPARATION COMPRISING A THERMALLY REDUCIBLE PLATINUM COMPOUND IN AN ORGANIC VEHICLE AND WHCIH ON HEATING TO A TEMPERATURE BETWEEN 350* AND 550* C. PRODUCES A DEPOSIT CONSISTING ESSENTIALLY OF PLATINUM, SAID PLATINUM COMPOUND BEING SELECTED FROM THE GROUP CONSISTING OF CHLOROPLATINIC ACID, BROMOPLATINIC ACID AND PLATINUM RESINATES, DRYING EACH COATING AND THEN FIRING THE SAME BY HEATING IN AN OXIDISING ATMOSPHERE TO A TEMPERATURE BETWEEN 350* AND 550* C. TO FORM SAID DEPOSIT CONSISTING ESSENTIALLY OF PLATINUM AND ELECTROLYSING AN AQUEOUS SOLUTION OF AN ALKALI METAL CHLORIDE IN AN ELECTROLYTIC CELL USING SAID PLATINUM COATED TITANIUM SUPPORT AS THE ANODE. 